This invention relates to improvements in walking dragline excavator frames, and is more particularly concerned with relieving unsymmetrical roller load stress in the rail pads and associated structure of such frames.
Since the introduction of large capacity walking dragline excavators in which the relatively rotatably related frames have single web rail girders backing up the rail pads of the frames, such frames have experienced unreasonably short service life due to cyclically varying, unbalanced loads imposed by the rollers of the rotative system operative between the frames. The loads of individual rollers often exceed 350,000 pounds and where the heavy roller loads are imposed eccentric to the rail girder and the welded joint of the rail girder to the rail pad, repeated, fatigue bending and torsional stresses cause serious fatigue failures of the joint.
In my prior U.S. Pat. Nos. 4,611,440 and 4,769,932 (which to any extent necessary are incorporated herein by reference) problems of existing frames are discussed and the rail pad/rail girder joint failure problem has been addressed by the provision of multi-web rail beam girders which satisfactorily solve the problem where such rail beam girders are part of the original equipment of the walking dragline excavator frames. However, due to the multiplication of the heavy duty plates of the multi-web rail girders, not only is the cost of the frames but also the total weight substantially increased. Single web rail girders in existing walking dragline excavator frames already in the field are extremely difficult to repair after relatively short life failure of the rail pads/rail beam welded joint.
Detailed evaluation of the single web rail pad/girder failures reveals that generally only the adjacent part of the rail girder, equal to about 1/6 of the total depth of the rail girder, is subjected to the high cyclical stresses which cause premature failure. The damaged part of the rail girder constitutes about 5% of the total weight of the base frame. However, in the field rewelding and repairing of the joint between the rail pad and the rail girder has proved generally unsuccessful. Replacement at high cost of the entire frame, and in particular the base frame, has been more convenient and economical than the presently available repair or replacement methods for the damaged part of the rail girder.